Shell Programming and Scripting

- A  H        C
- A  HHH    F
- AAA  HH   I

awk '$2=="AAA" && $3=="HH" {$4="IJ" } 1' myfile.txt

- A  H      C
- A  HHH    F
- AAA HH IJ

akshay@Aix:/tmp$ cat file
- A  H        C
- A  HHH    F
- AAA  HH   I

akshay@Aix:/tmp$ awk '$2=="AAA" && $3=="HH" {$0 = substr($0,1,index($0,$4)-1) "IJ"} 1' file
- A  H        C
- A  HHH    F
- AAA  HH   IJ

substr($0,1,index($0,$4)-1)

awk '$2=="AAA" && $3=="HH" {sub($4 "[[:blank:]]*$","IJ")} 1' file

index(in, find)
    This searches the string in for the first occurrence of the string find, and returns the position in characters where that occurrence begins in the string in. For example:
awk 'BEGIN { print index("peanut", "an") }'

prints `3'. If find is not found, index returns 0. (Remember that string indices in awk start at 1.) 

substr(string, start, length)
    This returns a length-character-long substring of string, starting at character number start. The first character of a string is character number one. For example, substr("washington", 5, 3) returns "ing". If length is not present, this function returns the whole suffix of string that begins at character number start. For example, substr("washington", 5) returns "ington". This is also the case if length is greater than the number of characters remaining in the string, counting from character number start.